1. Field of the Invention
This invention relates to a process for stretch wrapping of articles with thermoplastic film. In one aspect it relates to packaging of articles by stretch wrap pass through technique (herein referred to as "Pass-Through" process).
2. Description of the Prior Art
The packaging of articles by the Pass-Through process has been employed for several years to facilitate transportation and storage of bulky articles. The technique is normally carried out on articles stacked on a pallet wherein the loaded pallet is conveyed through a film dispensing and film stretching apparatus. As the pallet moves through the apparatus, the film is stretched and wrapped around the front and opposite sides of the loaded pallet and joined by heat seal on the rear side thereof. The goods thus are anchored to the pallet in an integrated body which facilitates transportation and storage, as well as providing protection for the goods.
The thermoplastic film used in such operations must possess strength as well as high degree of stretching and resist tear as the film is stretched around corners of the stacked goods. Films that have been used in such operations include conventional high pressure low density polyethylene and low pressure linear low density polyethylene. These films are normally used in a single layer and have sufficient thickness to permit substantial stretching.
In stretch wrapping by the Pass-Through process, most film failures occur at the top or bottom corners of the load, mainly due to the film's inability to slide around corners. If the film sticks or hangs up on a corner, it will most likely become punctured by the corner and tear in the transverse direction as the stretching tension is increased.
Stretch films usable in the Pass-Through process thus must possess the following properties: a relative high degree of stretch, transverse direction tear resistance, puncture resistance, and a medium slip.
A variety of thermoplastic films, including ethylene homo and copolymers and blends thereof, have been used in packaging by the Pass-Through process. These films, however, have been used in a single layer of sufficient thickness to provide the necessary strength properties identified above. The recent advent of commercial linear low density polyethylene (LLDPE) is particularly suited for stretch wrapping. The short chain branching of the LLDPE molecules permit the LLDPE film to be more easily stretched than polymers with long chain branching such as conventional high pressure polyethylene (LDPE).
As will be discussed in more detail below, the present invention employs a double layer of polyethylene film. The prior art which discloses the use of multi-layer films includes U.S. Pat. Nos. 4,160,053, 4,303,710, 4,228,215, 4,329,388, 4,294,889, 4,132,050, 4,297,411, and 4,258,848. These references, however, generally relate to laminates or blocked film layers. None of these references disclose the use of such films in the packaging operation using Pass-Through process.